An assembly line is a manufacturing process (often called a progressive assembly) in which parts (usually interchangeable parts) are added as the semi-finished assembly moves from workstation to workstation where the parts are added in sequence until the final assembly is produced. By mechanically moving the parts to the assembly work and moving the semi-finished assembly from work station to work station, a finished product can be assembled faster and with less labor than by having workers carry parts to a stationary piece for assembly.

Concepts

Assembly lines are designed for the sequential organization of workers, tools or machines, and parts. The motion of workers is minimized to the extent possible. All parts or assemblies are handled either by conveyors or motorized vehicles such as fork lifts, or gravity, with no manual trucking. Heavy lifting is done by machines such as overhead cranes or forklifts. Each worker typically performs one simple operation.

(1) Place the tools and the men in the sequence of the operation so that each component part shall travel the least possible distance while in the process of finishing.

(2) Use work slides or some other form of carrier so that when a workman completes his operation, he drops the part always in the same place—which place must always be the most convenient place to his hand—and if possible have gravity carry the part to the next workman for his own.

(3) Use sliding assembling lines by which the parts to be assembled are delivered at convenient distances.[2]

Designing assembly lines is a well-established mathematical challenge, referred to as assembly line balancing problem.[3] The aim is to assign a set of tasks which need to be performed on the workpiece to a sequence of workstations. Each task requires a given task duration for completion. The assignment of tasks to stations is typically limited by two constraints: (1) a precedence graph which indicates what other tasks need to be completed before a particular tasks can be initiated (e.g. not putting in a screw before drilling the hole) and (2) a cycle time which restricts the sum of task durations which can be completed at each workstation before the workpiece is moved to the next station by they conveyor belt. Major planning problems for operating assembly lines include supply chain integration, inventory control and production scheduling.[4]

Simple example

c.1984 Public Handout showing the GM Oshawa Car Assembly line and the stages of car manufacturing

Consider the assembly of a car: assume that certain steps in the assembly line are to install the engine, install the hood, and install the wheels (in that order, with arbitrary interstitial steps); only one of these steps can be done at a time. In traditional production, only one car would be assembled at a time. If engine installation takes 20 minutes, hood installation takes five minutes, and wheels installation takes 10 minutes, then a car can be produced every 35 minutes.

In an assembly line, car assembly is split between several stations, all working simultaneously. When a station is finished with a car, it passes it on to the next. By having three stations, three cars can be operated on at the same time, each at a different stage of assembly.

After finishing its work on the first car, the engine installation crew can begin working on the second car. While the engine installation crew works on the second car, the first car can be moved to the hood station and fitted with a hood, then to the wheels station and be fitted with wheels. After the engine has been installed on the second car, the second car moves to the hood assembly. At the same time, the third car moves to the engine assembly. When the third car's engine has been mounted, it then can be moved to the hood station; meanwhile, subsequent cars (if any) can be moved to the engine installation station.

Assuming no loss of time when moving a car from one station to another, the longest stage on the assembly line determines the throughput (20 minutes for the engine installation) so a car can be produced every 20 minutes, once the first car taking 35 minutes has been produced.

History

Before the Industrial Revolution, most manufactured products were made individually by hand. A single craftsman or team of craftsmen would create each part of a product. They would use their skills and tools such as files and knives to create the individual parts. They would then assemble them into the final product, making cut-and-try changes in the parts until they fit and could work together (craft production).

The Venetian Arsenal, dating to about 1104, operated similar to a production line. Ships moved down a canal and were fitted by the various shops they passed. At the peak of its efficiency in the early 16th century, the Arsenal employed some 16,000 people who could apparently produce nearly one ship each day, and could fit out, arm, and provision a newly built galley with standardized parts on an assembly-line basis. Although the Arsenal lasted until the early Industrial Revolution, production line methods did not become common even then.[6]

The automatic flourmill built by Oliver Evans in 1785 was called the beginning of modern bulk material handling by Roe (1916). Evans's mill used a leather belt bucket elevator, screw conveyors, canvas belt conveyors, and other mechanical devices to completely automate the process of making flour. The innovation spread to other mills and breweries.[8][9]

One of the earliest examples of an almost modern factory layout, designed for easy material handling, was the Bridgewater Foundry. The factory grounds were bordered by the Bridgewater Canal and the Liverpool and Manchester Railway. The buildings were arranged in a line with a railway for carrying the work going through the buildings. Cranes were used for lifting the heavy work, which sometimes weighed in the tens of tons. The work passed sequentially through to erection of framework and final assembly.[11]

The Bridgewater Foundry, pictured in 1839, one of the earliest factories to use an almost modern layout, workflow, and material-handling system

The first flow assembly line was initiated at the factory of Richard Garrett & Sons, Leiston Works in Leiston in the English county of Suffolk for the manufacture of portable steam engines. The assembly line area was called 'The Long Shop' on account of its length and was fully operational by early 1853. The boiler was brought up from the foundry and put at the start of the line, and as it progressed through the building it would stop at various stages where new parts would be added. From the upper level, where other parts were made, the lighter parts would be lowered over a balcony and then fixed onto the machine on the ground level. When the machine reached the end of the shop, it would be completed.
[12]

Late 19th-century steam and electric conveyors

Steam powered conveyor lifts began being used for loading and unloading ships some time in the last quarter of the 19th century.[13] Hounshell (1984) shows a c. 1885 sketch of an electric powered conveyor moving cans through a filling line in a canning factory.

The meatpacking industry of Chicago is believed to be one of the first industrial assembly lines (or dis-assembly lines) to be utilized in the United States starting in 1867. Workers would stand at fixed stations and a pulley system would bring the meat to each worker and they would complete one task. Henry Ford and others have written about the influence of this slaughterhouse practice on the later developments at Ford Motor Company.[14]

20th century

1913 Experimenting with mounting body on Model T chassis. Ford tested various assembly methods to optimize the procedures before permanently installing the equipment. The actual assembly line used an overhead crane to mount the body.

At Ford Motor Company, the assembly line was introduced by William "Pa" Klann upon his return from visiting Swift & Company's slaughterhouse in Chicago and viewing what was referred to as the "disassembly line", where carcasses were butchered as they moved along a conveyor. The efficiency of one person removing the same piece over and over without himself moving caught his attention. He reported the idea to Peter E. Martin, soon to be head of Ford production, who was doubtful at the time but encouraged him to proceed. Others at Ford have claimed to have put the idea forth to Henry Ford, but Pa Klann's slaughterhouse revelation is well documented in the archives at the Henry Ford Museum[19] and elsewhere, making him an important contributor to the modern automated assembly line concept. Ford was appreciative, having visited the highly automated 40-acre Searsmail order handling facility around 1906. At Ford, the process was an evolution by trial and error[16] of a team consisting primarily of Peter E. Martin, the factory superintendent; Charles E. Sorensen, Martin's assistant; Clarence W. Avery; C. Harold Wills, draftsman and toolmaker; Charles Ebender; and József Galamb. Some of the groundwork for such development had recently been laid by the intelligent layout of machine tool placement that Walter Flanders had been doing at Ford up to 1908.

In 1922, Ford (through his ghostwriter Crowther) said of his 1913 assembly line:

I believe that this was the first moving line ever installed. The idea came in a general way from the overhead trolley that the Chicago packers use in dressing beef.[23]

Charles E. Sorensen, in his 1956 memoir My Forty Years with Ford, presented a different version of development that was not so much about individual "inventors" as a gradual, logical development of industrial engineering:

What was worked out at Ford was the practice of moving the work from one worker to another until it became a complete unit, then arranging the flow of these units at the right time and the right place to a moving final assembly line from which came a finished product. Regardless of earlier uses of some of these principles, the direct line of succession of mass production and its intensification into automation stems directly from what we worked out at Ford Motor Company between 1908 and 1913. Henry Ford is generally regarded as the father of mass production. He was not. He was the sponsor of it.[24]

As a result of these developments in method, Ford's cars came off the line in three-minute intervals, or six feet per minute.[25] This was much faster than previous methods, increasing production by eight to one (requiring 12.5 man-hours before, 1 hour 33 minutes after), while using less manpower.[7] It was so successful, paint became a bottleneck. Only japan black would dry fast enough, forcing the company to drop the variety of colors available before 1914, until fast-drying Ducolacquer was developed in 1926.[7]

The assembly line technique was an integral part of the diffusion of the automobile into American society. Decreased costs of production allowed the cost of the Model T to fall within the budget of the American middle class. In 1908, the price of a Model T was around $825, and by 1912 it had decreased to around $575. This price reduction is comparable to a reduction from $15,000 to $10,000 in dollar terms from the year 2000. In 1914, an assembly line worker could buy a Model T with four months' pay.[7]

Ford's complex safety procedures—especially assigning each worker to a specific location instead of allowing them to roam about—dramatically reduced the rate of injury. The combination of high wages and high efficiency is called "Fordism", and was copied by most major industries. The efficiency gains from the assembly line also coincided with the take-off of the United States. The assembly line forced workers to work at a certain pace with very repetitive motions which led to more output per worker while other countries were using less productive methods.

In the automotive industry, its success was dominating, and quickly spread worldwide. Ford France and Ford Britain in 1911, Ford Denmark 1923, Ford Germany and Ford Japan 1925; in 1919, Vulcan (Southport, Lancashire) was the first native European manufacturer to adopt it. Soon, companies had to have assembly lines, or risk going broke by not being able to compete; by 1930, 250 companies which did not had disappeared.[7]

The massive demand for military hardware in World War II prompted assembly-line techniques in shipbuilding and aircraft production. Thousands of Liberty Ships were built making extensive use of prefabrication, enabling ship assembly to be completed in weeks or even days. After having produced fewer than 3,000 planes for the United States Military in 1939, American aircraft manufacturers built over 300,000 planes in World War II. Vultee pioneered the use of the powered assembly line for aircraft manufacturing. Other companies quickly followed. As William S. Knudsen (having worked at Ford,[16] GM and the National Defense Advisory Commission) observed, "We won because we smothered the enemy in an avalanche of production, the like of which he had never seen, nor dreamed possible."[26][27]

Improved working conditions

In his 1922 autobiography,[2] Henry Ford mentions several benefits of the assembly line including:

Workers do not do any heavy lifting.

No stooping or bending over.

No special training was required.

There are jobs that almost anyone can do.

Provided employment to immigrants.

The gains in productivity allowed Ford to increase worker pay from $1.50 per day to $5.00 per day once employees reached three years of service on the assembly line. Ford continued on to reduce the hourly work week while continuously lowering the Model T price. These goals appear altruistic; however, it has been argued that they were implemented by Ford in order to reduce high employee turnover: when the assembly line was introduced in 1913, it was discovered that "every time the company wanted to add 100 men to its factory personnel, it was necessary to hire 963" in order to counteract the natural distaste the assembly line seems to have inspired.[28]

Sociological problems

One of capitalism's most famous critics, Karl Marx, expressed in his Entfremdung theory the belief that in order to achieve job satisfaction workers need to see themselves in the objects they have created, that products should be "mirrors in which workers see their reflected essential nature." Marx viewed labour as a chance for us to externalize facets of our personality. Marxists argue that specialization makes it very difficult for any worker to feel they may be contributing to the real needs of humanity. The repetitive nature of specialized tasks causes, they say, a feeling of disconnection between what a worker does all day, who they really are, and what they would ideally be able to contribute to society. Marx also argued that specialised jobs are insecure, since the worker is expendable as soon as costs rise and technology can replace more expensive human labour.[30]

^ abcdWeber, Austin (2013-10-01). "The Moving Assembly Line Turns 100". Assembly Magazine. Archived from the original on 2016-08-26. Retrieved 2017-03-26. The assembly line ... was the result of a long period of trial and error. The assembly line wasn't a planned development; rather, it emerged in 1913 from a dynamic situation. People such as Carl Emde, William Klann and William Knudsen all played key roles in early automation efforts at Ford's Highland Park factory. Two individuals were essential to the success of the moving assembly line: Clarence Avery and Charles Sorensen. constant redesign of the Model T. Many components were tweaked regularly to make the vehicle easier to assemble. In 1913 alone, Ford made more than 100 design changes every month. Continuous experimentation was the rule rather than the exception at Ford's Highland Park plant. Ford engineers were constantly redesigning and tweaking jigs and fixtures, and planning new machine tools or fixing old ones, to achieve higher production.